Historically, military and other high-reliability devices have been hermetically packaged in ceramic parts. A gold-silicon eutectic bonding process has been used to bond the integrated circuit (IC) to the header (ceramic substrate) to avoid entrapped and evolved moisture and other gasses typically experienced when using silver-filled organic polymers in plastic packages.
The primary reasons for developing an alternative to gold-eutectic are the need for lower cost processing, faster processing, less stress transmissions to the IC as larger chips are designed, and improved heat transfer.
To meet these needs, the most suitable approach now recognized is a conductive metal-filled glass system that contains no organic material after processing. Silver is the metal of choice because of industry acceptance in epoxy systems typically used in plastic packages. Silver has the necessary properties and is available in suitable flake size and purity.
The silver-glass matrix provides a more void-free bonding layer than the gold-silicon eutectic system. Thus, although the thermal conductivity of gold is greater than that of the silver-glass, the absence of voids in the matrix provides greater heat transfer.
As ICs become larger, they are more susceptible to cracking due to stress induced by mismatched thermal coefficients of expansion (TCE) with the package. The silver-glass matrix transmits less stress to the IC than the gold-silicon layer.